Brenda Morley, age 14, of Brownsville, Texas, for her question:
WHAT EXACTLY IS A GALAXY?
A galaxy is a massive group of hundreds of millions of stars, all gravitationally interacting and orbiting about a common center. All of the stars visible to the unaided eye from Earth belong to the Earth's galaxy, which is called the Milky Way.
In addition to stars and planets, galaxies also contain star clusters, atomic hydrogen gas, molecular hydrogen, cosmic rays and complex molecules composed of hydrogen, nitrogen, carbon and silicon.
First to describe the spiral galaxy was a Persian astronomer named al Sufi who lived in the first century. By the middle of the 18th century, only three galaxies had been identified but by 1780 astronomers were able to list 32 of them.
Thousands of galaxies were identified and catalogued by British astronomers Sir William and Caroline Herschel and Sir John Herschel during the early part of the 19th century. Since 1900 galaxies have been discovered in large numbers by photographic searches.
Galaxies at enormous distances from Earth appear so tiny on a photograph that they can hardly be distinguished from stars.
Astronomers found evidence to indicate that all galaxies are moving away from one another and it led them to the conclusion that the universe is expanding. it is not known if the universe will continue to expand or if it contains sufficient matter to slow down the galaxies gravitationally so they will eventually begin contracting to the point from which they arose.
When viewed or photographed with a large telescope, only the nearest galaxies exhibit individuals stars. For most galaxies, only the combined light of all the stars is detected.
In viewing a galaxy with a telescope, inferring its distance is often impossible, for it may be a gigantic galaxy at a large distance or a smaller one closer to Earth.
Distance is estimated by comparing the brightness of objects in unknown galaxies with those in the earth's galaxy.
Galaxies are generally not isolated in space but are often members of small or moderate sized groups, which in turn from large clusters of galaxies. Earth's galaxy is one of a small group of about 20 galaxies that astronomers call the Local Group.
Earths galaxy and the Andromeda galaxy are the two largest members of the Local Group, each with a million million stars.
The Large, Small and Mini Magellanic Clouds are nearby satellite galaxies, but each is small and faint, with about 100 million stars.
The nearest cluster is the Virgo cluster. The Local Group is an outlying member of this cluster. The cluster contains thousands of galaxies arranged in a loosely spherical shape.
Clusters of galaxies also group to form mammoth superclusters with dimensions up to 300 million light years across. Overall, the distribution of clusters and superclusters in the universe is not uniform. Instead, superclusters of tens of thousands of galaxies are arranged in long, stringy, lace like filaments that are arranged around large voids. Large volumes of space in the universe contain few or no galaxies.